1. Field of the Invention
The present invention relates to an image pickup apparatus that performs autofocus (AF) control in response to an output of an image pickup device, a control method therefore, and a storage medium that stores a control program therefor.
2. Description of the Related Art
In general, a focus detection method with imaging surface phase difference detection using focus detection pixels of an image pickup device is known as one of focus detection methods in an image pickup apparatus like a digital still camera.
For example, an image pickup device that has at least a pair of sensors (i.e., photoelectric conversion elements) for a micro lens provided for each pixel is known as an image pickup device that performs the imaging surface phase difference detection (see Japanese laid-open patent publication (Kokai) No. S58-24105 (JP S58-24105)).
In the focus detection with the imaging surface phase difference detection, a pair of image signals in different pupil areas are used, and a correlation operation process by which a correlative value between the image signals is calculated one by one while shifting the image signals relatively is performed. An in-focus position is found according to a shift amount with the highest correlative value as a defocus amount. Then, a focusing control is performed according to the defocus amount concerned.
FIG. 15A and FIG. 15B are views showing focus detection by the imaging surface phase difference detection in a conventional image pickup apparatus. FIG. 15A is a view showing a focus detection area and shift areas that are arranged in an image pickup area. FIG. 15B is a view showing a settable range of the focus detection area. It should be noted that the focus detection area (A) and the shift areas (B) are collectively called a correlation operation area in the following description.
In the example shown in FIG. 15A, the focus detection area (A) 1502 is arranged in the center of a screen 1501. The shift areas (B) 1503 that are used for reading and correlation operations are adjacently arranged at both sides of the focus detection area (A) 1502 in a horizontal direction.
In the correlation operation process, adjacent areas of the focus detection area (A) 1502 are secured as the shift areas (B) 1503 in addition to the focus detection area (A) 1502, and the shift process of a predetermined amount (an amount corresponding the twice of the width of the shift area (B) 1503 in this example) is performed. Then, whenever the shift process is performed, the correlation operation is performed to calculate the defocusing amount for the focusing control.
Incidentally, since the shift areas (B) 1503 are set at the both sides of the focus detection area (A) 1502, a settable range 1504 (indicated by an alternate long and short dash line in FIG. 15B) within which the focus detection area (A) 1502 is settable becomes narrower than the screen 1501, which is an image pickup area, by the width of the shift area (B) 1503 (see FIG. 15B).
As mentioned above, in the conventional image pickup apparatus, a correlation operation process cannot be always performed in the entire image pickup area because the settable range of the focus detection area is restricted in the image pickup area. Accordingly, the conventional image pickup apparatus is difficult to perform the focusing control with sufficient accuracy over the entire image pickup area.